Chronostratigraphy
From Wikipedia, the free encyclopedia
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.
The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages (biostratigraphy). The aim of chronosratigraphy is to give a meaningful age date to these fossil assemblage intervals and interfaces.
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[edit] Methodology
Chronostratigraphy relies heavily upon isotope geology and geochronology to derive hard dating of known and well defined rock units which contain the specific fossil assemblages defined by the stratigraphic system. However it is practically very difficult to isotopically date most fossils and sedimentary rocks directly, and thus inferences must be made in order to arrive at an age date which reflecs the beginning of the interval.
The methodology used is derived from the law of superposition and the principles of cross-cutting relationships.
Because igneous rocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they contain mineral assemblaged which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column will rely heavily on intrusive and extrusive igneous rocks.
Metamorphism, often associated with faulting, may also be used to bracket depositional intervals in a chronostratigraphic column. Metamorphic rocks can, occasionally, be dated, and this may give some limits to the age at which a bed could be laid down. For example, if a bed containing graptolites overlies crystalline basement at some point, dating the crystalline basement will give a maximum age of that fossil assemblage.
However, this process requires a considerable degree of effort and checking of field relationships and age dates. For instance, there may be many millions of years between a bed being laid down and an intrusive rock cutting it; the estimate of age must necessarily be between the oldest cross-cutting intrusive rock in the fossil assemblage and the youngest rock upon which the fossil assemblage rests.
[edit] Units
Chronostratigraphic units, with examples:
- eonothem - Phanerozoic
- erathem - Paleozoic
- system - Ordovician
- series - Upper Ordovician
- stage - Ashgill
[edit] Chronostratigraphy at a Glance
— Chronostratigraphic units refers to the rocks in quarries, drill cores, seismographic charts etc.
— Rocks of the Upper Jurassic Series were formed during the Late Jurassic Epoch.
— You can write: Tyrannosaurus rex was found in Upper Cretaceous, but if you write:"the T-rex was living in Upper Cretaceous", you mean that the T-rex was living inside the rock!
Chronostratigraphy is such an important branch of stratigraphy, if not the most important, because the age correlations derived are crucial to drawing accurate cross sections of the spatial organization of rocks and to prepare accurate paleogeographic reconstructions.
